A typical electron beam emitter includes a vacuum chamber with an electron generator positioned therein for generating electrons. The electrons are accelerated out from the vacuum chamber through an exit window in an electron beam. Typically, the exit window is formed from a metallic foil. The metallic foil of the exit window is commonly formed from a high strength material such as titanium in order to withstand the pressure differential between the interior and exterior of the vacuum chamber.
A common use of electron beam emitters is to irradiate materials such as inks and adhesives with an electron beam for curing purposes. Other common uses include the treatment of waste water or sewage, or the sterilization of food or beverage packaging. Some applications require particular electron beam intensity profiles where the intensity varies laterally. One common method for producing electron beams with a varied intensity profile is to laterally vary the electron permeability of either the electron generator grid or the exit window. Another method is to design the emitter to have particular electrical optics for producing the desired intensity profile. Typically, such emitters are custom made to suit the desired use.
The present invention is directed to a filament for generating electrons for an electron beam emitter in which the configuration of the filament is varied for producing a desired electron generation profile. Consequently, a standardized electron beam emitter may be used for a variety of applications requiring different intensity profiles with the configuration of the filaments within the emitter being selected to provide the desired electron beam intensity profile.
In preferred embodiments, the filament has a cross section and a length. The cross section of the filament is varied along the length for producing a desired electron generation profile. Typically, the filament has varying cross sectional areas along the length. In situations where the cross section of the filament is round, the filament also has varying diameters along the length. Consequently, the filament can have at least one major cross sectional area (or major diameter) and at least one minor cross sectional area (or minor diameter). The major cross sectional area (or major diameter) is greater than the minor cross sectional area (or minor diameter). The at least one minor cross sectional area (or minor diameter) increases temperature and electron generation at the at least one minor cross sectional area (or minor diameter). The filament can have multiple minor cross sectional areas or minor diameters which are spaced apart from each other at selected intervals.
In one embodiment, the at least one minor cross sectional area or minor diameter is positioned at or near one end of the filament to compensate for voltage drop across the length of the filament so that the filament is capable of uniformly generating electrons along the length of the filament. In another embodiment, the at least one minor cross sectional area or minor diameter is positioned at or near opposite ends of the filament for generating a greater amount of electrons at or near the ends.
Typically, the filament is part of an electron generator which is positioned within a vacuum chamber of an electron beam emitter. The vacuum chamber has an exit window through which the electrons generated by the filament exit the vacuum chamber in an electron beam.
In the present invention, by varying the cross sectional areas or diameters of the electron generating filament, a variety of desired electron generation profiles can be selected to suit specific applications. Since no significant changes need to be made to the components of an electron beam emitter including such a filament, and fabrication of the filament is relatively inexpensive, the cost of an electron beam emitter employing the filament is not greatly increased.